1. Field of the Invention
This invention relates to multi-axis sensing devices and more particularly to vibrating structure gyroscopes that employ a resonant element.
2. Discussion of Prior Art
Vibrating structure gyroscopes have been fabricated using a variety of structures for the resonant element including beams tuning forks, cylinders and rings. Aside from measurement of the rate of rotation about a particular axis, these devices are also capable of operation in xe2x80x9cwhole anglexe2x80x9d or xe2x80x9cgyroscopexe2x80x9d mode in which the device output gives a direct measure of the angle of rotation about a particular axis, as described in U.S. Pat. No. 5,218,867. This mode of operation is known to give advantages in terms of improved scalefactor performance, particularly in applications where the device is subjected to sustained high rates of rotation.
Planar rings have been shown to be particularly versatile, with single-axis rate zero variants being commercially available using both conventionally fabricated and micro-machined resonators.
Conventional single-axis planar ring gyroscopes typically use cos 2xcex8 in-plane vibration mode pairs. For a perfectly symmetric resonator there will be two degenerate modes at a mutual angle of 45xc2x0. These are shown schematically in FIG. 1a (cos 2xcex8 mode) and FIG. 1b (sin 2xcex8 mode) which show the ring distortion at the two extremes of motion during a single vibration cycle. One of these modes is excited as the carrier mode (FIG. 1a). When the structure is rotated about the axis normal to the plane of the ring (the z-axis). Coriolis forces couple energy into the response mode (FIG. 1b). The Coriolis force, and hence the amplitude of the response mode motion is directly proportional to the applied rotation rate. Other higher-order cos nxcex8 mode pairs may also be used in similar fashion.
In operation, the carrier mode is driven at the resonance maximum and is typically maintained at a constant amplitude. The Coriolis forces generated as a result of rotation will be at the carrier resonance frequency. The response mode frequency is typically matched to that of the carrier and thus the motion arising as a result of these forces is amplified by the Q (quality factor) of the structure, giving enhanced sensitivity. This response mode motion may be nulled using a force feedback loop with the nulling force then being directly proportional to the applied rate. This mode of operation removes the Q dependence from the rate output and gives improved scalefactor performance. The motion of the ring is thus maintained at a fixed angular orientation at all times.
Planar ring structures are also suitable for use in single-axis attitude sensors using in-plane cos nxcex8 mode pairs, such as described in relation to a cylindrical element in U.S. Pat. No. 5,218,867. In this mode of operation, the vibration energy is free to transfer between the in-plane mode pairs as the device is rotated, with no force feedback being applied. If the mode frequencies are accurately matched, this will be equivalent to the mode rotating around the ring as the structure is rotated. The mode pattern orientation is not inertially stable but tends to lag behind the rotation of the ring structure. The ratio of the pattern angle rotation to the applied rotation angle is given by an inertial coupling constant, K, which is dependent upon the resonator structure and the mode order, n.
When operating in this mode, the same drive and pick-off configurations may be employed as for conventional closed-loop rate gyro operation. The techniques for detecting the mode orientation on the ring and for maintaining the amplitude of motion are, however, significantly different. A radial drive signal is applied to sustain the vibration amplitude at one or more of the radial anti-nodes. As the mode pattern rotates around the ring, the effective drive position is required to track the radial anti-node around the ring. The pick-offs must similarly have the capability of resolving the actual radial motion of the ring at both the radial anti-node and node. The radial anti-node signal is used to maintain the drive frequency at the resonance maximum and to normalise the vibration amplitude. The radial node signal is required to track the mode position accurately.
Ring structures are also capable of providing rate sensitivity around multiple axes, as described in UK Patent Application Nos. 2318184A and 2335273A. When driven in a cos nxcex8 in-plane carrier mode, rotations about axes in the plane of the ring will also give rise to Coriolis forces. These forces will be along the axis normal to the plane of the ring (z-axis). For rotation around the y-axis xcexa9y, where the y-axis is taken to be along xcex8=0xc2x0, these Coriolis forces, Fz(xcex8), will have an angular distribution given by:
Fz(xcex8)=Fn+1xcexa9y sin(n+1)xcex8+Fnxe2x88x921xcexa9y sin(nxe2x88x921)xcex8
where xcex8 is the angular position around the ring with respect to a fixed reference position, n is the carrier mode order and the parameters Fxe2x88x921 and Fn+1 are constants which depend on the precise geometry of the ring, the material from which the ring is made, and the value of n. Similarly, for rotation about the x-axis, xcexa9x, the Coriolis forces will have an angular distribution given by:
Fz(xcex8)=Fnxe2x88x921xcexa9x cos(n+1)xcex8xe2x88x92Fnxe2x88x921xcexa9x cos(nxe2x88x921)xcex8
These forces thus have components that are capable of directly exciting either the sin(nxe2x88x921)xcex8 and cos(n+1)xcex8 or the sin(nxe2x88x921)xcex8 and cos(nxe2x88x921)xcex8 out-of-plane mode pairs. The rin dimensions may be set such that the resonant frequency of one of the mode pairs exactly matches that of the in-plane carrier mode. In this way, the amplitude of the out-of-plane response motion will be amplified by the Q of the structure as for the in-plane response.
Using such designs, a single device can provide all the functionality required for navigation applications where previously two or three single-axis devices would be needed, one device being dedicated to each axis. Multi-axis devices have the advantage that the mutual alignment of the sensing axes is set during the resonator fabrication process, and yet single-frequency operation for all axes means that the electronics need not be significantly more complex than in a single-axis device. For applications requiring rate sensitivity around multiple axes, such a multi-axis device may provide a significant reduction in both cost and size.
Between them, UK Patent Application Nos. 2318184A and 2335273A describe various modal combinations that may be employed to implement both two- and three-axis rate gyroscopes. The locations of the drive and pick-off transducer elements appropriate for each combination are also shown therein.
The disclosures of U.S. Pat. No. 5,218,867 and UK Patent Application Nos. 2318184A and 2335273A are incorporated herein by reference.
Certain gyroscope applications may require measurement of the spatial orientation of a body that is subject to high rates of rotation about one particular axis. In aircraft navigation, for example, the roll axis of the aircraft may be subject to higher rotation rates than the pitch and yaw axes. In order to compute the orientation in the pitch and yaw axes in such applications, it is essential that the orientation in the roll axis is known to a high degree of accuracy.
Consequently, for axes experiencing high rotation rates, there is a considerable performance advantage in operating in xe2x80x9cwhole anglexe2x80x9d mode to prevent the accumulation of a heading error due to scalefactor error. By way of illustration, a 1% scalefactor error will result in a 3.6xc2x0 heading error for each revolution. This problem of cumulative error is particularly acute when sensing the motion of a wheel or axle, in which one axis will obviously experience vastly higher rotation rates than the other two axes.
There is therefore a requirement for a device that combines the advantages of multi-axis operation whilst providing for accurate measurement of orientation around a single axis that may be subject to sustained high rates of rotation.
The present invention results from the insight that it is possible to operate the z-axis response of a multi-axis gyroscope in xe2x80x9cwhole anglexe2x80x9d or xe2x80x9cgyroscopexe2x80x9d mode whilst retaining the x- and y-axis responses in rate gyro mode. Accordingly, the invention may be expressed broadly as a three-axis gyroscopic sensing device adapted for operation as a rate gyroscope about two axes and as a whole angle gyroscope about the third axis.
The invention therefore resides in a vibrating structure gyroscope comprising a resonant body, drive transducer means for driving resonant motion of the body, pick-off means for producing signals representive of the resonant motion, and signal processing means for extracting z-axis orientation information and x- and y-axis rate information from the signals, such that the gyroscope operates as a whole angle gyroscope for rotations about the z-axis and as a rate angle gyroscope for rotations about the x-axis and y axis.
More specifically, the signal processing means extracts z-axis carrier mode orientation information from the signals and normalises this information to give information on the angular orientation about the z-axis, as well as extracting x- and y-axis rate information from the signals.
In such a gyroscope the resonant body is typically a planar ring structure and the resonant motion takes place in a vibration mode pattern in the plane of the ring whose orientation angle with respect to the body varies proportionately with the orientation of the body about its z-axis. This vibration mode pattern couples energy into an out-of-plane response mode motion in accordance with rotation of the body about the x- or y-axis. In this case, the signal processing means advantageously resolves the out-of-plane response mode motion with reference to a z-axis orientation signal representative of the orientation about the z-axis to extract the x- and y-axis rate information.
The pick-off means suitably comprises a first plurality of pick-offs positioned to sense resonant motion in the plane of the body and a second plurality of pick-offs positioned to sense response mode motion out of the plane of the body. The pick-offs of the second plurality should be separated by 30 kxc2x0, where k is an odd integer.
The drive transducer means preferably comprises a plurality of drive transducers driven via a drive resolver that takes input from the z-axis carrier vibration mode orientation signal to give a resultant drive resolved along the orientation angle of the vibration mode pattern.
In preferred embodiments of the invention, the signal processing means includes rate integration means that takes input signals from the first plurality of in-plane pick-offs via a pick-off resolver and outputs the z-axis orientation signal, and an x- y-axis resolver that takes as input drive signals applied to a plurality of out-of-plane drives, resolves those signals with reference to the z-axis carrier vibration mode orientation signal, and outputs the x- and y-axis rate information.
Advantageously, an anti-nodal signal from the pick-off resolver is applied to a phase locked loop that adjusts the drive frequency of the drive transducer means to track a resonance maximum. The anti-nodal signal is preferably also applied to a gain control loop that adjusts the drive level applied to the drive transducer means to maintain a constant anti-nodal signal.
A nodal signal from the pick-off resolver may be applied to a rate integration means that preferably comprises a phase detector to resolve any signal component that is in-phase with an anti-nodal signal. The rate integration means advantageously comprises a rate signal generator means such as a loop controller that takes the nodal signal and outputs a rate signal proportional to the rate of rotation of the vibration mode pattern about the z-axis. The rate integration means can then further comprise an integrator that integrates the rate signal to output the z-axis carrier vibration mode orientation signal. This z-axis carrier vibration mode orientation signal can be applied to a normalising means that applies the Bryan factor to that signal to give a direct measure of the angle through which the gyroscope body has rotated around the z-axis.
Conveniently, the pick-off means comprises an x-axis pick-off whose output is applied to an x-axis rate loop and a y-axis pick-off whose output is applied to a y-axis rate loop and the x- and y-axis rate loops respectively apply drive signals to x- and y-axis drive transducers to null the signal at the respective pick-offs.
To minimise vibration pattern drift, it is advantageous to employ a quadrature nulling loop. This loop preferably applies a drive signal to the drive transducer means along a nodal axis to maintain the input to the loop at zero.